Part of a known near field communication (NFC) system is shown schematically at 10 in FIG. 1. In the system of FIG. 1 a transmitter section of an NFC reader 12 comprises a voltage source power amplifier 14 having differential outputs that are connected to input terminals of an antenna 16. Capacitances 18a, 18b are connected in series between the outputs of the power amplifier 14 and the input terminals of the antenna 16. A further capacitance 20 is connected in parallel between the outputs of the power amplifier 14 and the antenna 16, whilst resistances 22a, 22b are connected in series between the capacitances 18a, 18b and the input terminals of the antenna 16. The capacitances 18a, 18b, 20 and resistances 20a, 20b form, with the inductance of the antenna 16, a mainly series resonant circuit.
An NFC tag 24 communicates with the reader 12 by means of an antenna 26, with the other components of the tag 24 being represented by a capacitor 28 and a resistor 30 connected in parallel with the antenna 26.
The power amplifier 14, capacitances 18a, 18b, 20 and resistances 22a, 22b may be implemented as part of an integrated circuit (i.e. may be “on-chip” components), whilst the antenna 16 is an off-chip component (i.e. it is external to the integrated circuit containing the power amplifier 14, capacitances 18a, 18b, 20 and resistances 22a, 22b).
The resonant frequency of the resonant network formed from the capacitances 18a, 18b, 20, resistances 22a, 22b and the self-inductance of the antenna 16 is determined at least in part by the value of the capacitances 18a, 18b, 20. For optimum transmission of data it is important that the resonant frequency of the parallel resonant circuit is equal to, or at least very close to, the frequency of the signal to be transmitted by the reader 12.
As will be appreciated by those skilled in the art, the reader 12 of the NFC system illustrated in FIG. 1 uses a series resonant antenna 16. This is required because only a series resonant antenna is able to power the external passive tag 24. However, the use of the series resonant antenna 16 limits the magnetic field strength that can be achieved by the antenna 16 of the reader 12 for the current through the antenna 16 that creates a magnetic field can never be greater than the current output by the power amplifier 14.